1. Field of the Invention
The present invention relates to a method, device, and system for waveform shaping of signal light.
2. Description of the Related Art
A Mach-Zehnder interferometer (MZI) type optical gate is known as a conventional waveform shaping device for performing waveform shaping on the optical level. This optical gate is configured by integrating a Mach-Zehnder interferometer including first and second nonlinear optical media each for providing a phase shift on an optical waveguide substrate, for example. Probe light as continuous wave (CW) light is equally divided into two components, which are in turn supplied to the first and second nonlinear optical media. The optical path length of the interferometer is set so that output light is not obtained by interference of the two components of the probe light.
An optical signal is further supplied to one of the first and second nonlinear optical media. By properly setting the powers of the optical signal and the probe light, a converted optical signal synchronous with the optical signal is output from the optical gate. The converted optical signal has the same wavelength as that of the probe light.
It has been proposed to use a semiconductor optical amplifier (SOA) as each of the first and second nonlinear optical media. For example, an InGaAs-SOA having end faces treated with antireflection coatings is used as each nonlinear optical medium in a 1.5 μm band, and these nonlinear optical media are integrated on an InP/GaInAsP substrate to fabricate an optical gate.
A nonlinear optical loop mirror (NOLM) is known as another conventional waveform shaping device. The NOLM includes a first optical coupler including first and second optical paths directionally coupled to each other, a loop optical path for connecting the first and second optical paths, and a second optical coupler including a third optical path directionally coupled to the loop optical path.
By forming a part or the whole of the loop optical path from a nonlinear optical medium and supplying probe light and an optical signal respectively to the first optical path and the third optical path, a converted optical signal is output from the second optical path.
An optical fiber is generally used as the nonlinear optical medium in the NOLM. In particular, a NOLM using a SOA as the nonlinear optical medium is referred to as an SLALOM (Semiconductor Laser Amplifier in a Loop Mirror).
In an optical fiber communication system that has been put to practical use in recent years, a reduction in signal power due to transmission line loss, coupling loss, etc. is compensated by using an optical amplifier such as an erbium doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier, which functions to linearly amplify a signal. In this kind of optical amplifier, amplified spontaneous emission (ASE) noise generated in association with the amplification is added to cause a reduction in signal-to-noise ratio (S/N ratio), so that the number of repeaters is limited to result in the limit of a transmission distance. Further, waveform degradation due to the chromatic dispersion owned by an optical fiber and the nonlinear optical effects in the fiber is another cause of the transmission limit. To break down such a limit, a regenerative repeater for digitally processing a signal is required. In particular, an all-optical regenerative repeater capable of performing all kinds of signal processing in optical level is important in realizing a transparent operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all-optical regenerative repeater are amplitude restoration or reamplification, waveform shaping or reshaping, and timing restoration or retiming. These functions are referred to as 3R functions, and in particular, the first and second functions are referred to as 2R functions.
The 2R functions can be provided by combining a waveform shaping device and an optical amplifier, or by using a waveform shaping device having an optical amplifying function. Further, the 3R functions can be provided by additionally using a clock regenerator in parallel to the 2R functions.
The present inventors have already proposed a waveform shaping device for providing the 2R functions and/or the 3R functions (Japanese Patent Application No. Hei 11-293189). In this device, two NOLMs are combined to thereby increase the degree of freedom of wavelength conversion in the case of obtaining a function of waveform shaping or optical gate.
Particularly in the waveform shaping device for providing the 2R functions and/or the 3R functions, there is a case that sufficient 3R functions cannot be obtained according to the degree of deterioration of an optical signal on which the extraction of a clock pulse in the clock regenerator is based.
Further, in the case of applying wavelength division multiplexing (WDM) to dramatically increase a transmission capacity, it is expected that the waveform shaping device for providing the 2R functions and/or the 3R functions may be complicated according to the number of WDM channels. Accordingly, a waveform shaping device suitable for WDM is demanded.